In recent years, many kinds of flat type display devices have been proposed and are put in practical use. Among them, display devices using electroluminescence (hereinafter abbreviated as “EL”) devices as a light emitting device of the surface-emitting type have received attention because of their usefulness. The EL devices are used as a backlight in liquid crystal displays or used for matrix type display devices in which the EL devices themselves are arranged in an array. For example, matrix type display devices using EL devices have good features such as self-luminosity, excellent visibility, wide view angle, and fast response. However, EL devices using organic materials as a luminescent material are insufficient in long-term reliability for the use in display devices. Furthermore, inorganic EL devices using inorganic materials as a luminescent material are insufficient in brightness and efficiency. Because of these and other reasons, the use of the EL devices is limited to specific applications.
Light emitting diodes (hereinafter abbreviated as “LED”) put in practical use as a high brightness and high-efficiency light source also can be included in EL devices in a broad sense. The widespread use of the LEDs is now growing because the light emitting devices that emit high brightness blue light or high brightness green light have been developed. However, the LEDs are put in practical use only as a point light source, and use of the LEDs in display devices is limited to a specific application such as a light source for a backlight in liquid crystal displays.
As a material of the LEDs, group 13 nitride semiconductors have received attention. The group 13 nitride semiconductors have a wide band gap and emit light in the range from the ultraviolet region to the visible light region in accordance with their compositions. Furthermore, the group 13 nitride semiconductors are direct transition type semiconductors and have an effective energy band structure as a light emitting material. However, gallium nitride (GaN), a typical example of the group 13 nitride semiconductors, has the disadvantage of a substrate material being restricted due to a difference in a lattice constant and a difference in a thermal expansion coefficient. Although sapphire substrates mostly are used, their lattice mismatch ratios are about 1000 times larger than those of other semiconductor devices, and the resultant threading dislocation density is five orders of magnitude higher. That is, the application to large-area display devices is still difficult in view of performance and in view of cost.
In order to overcome such drawbacks of the LEDs, techniques using particulate or columnar group 13 nitride semiconductors have been invented. These techniques were invented for the purpose of forming p-n junctions, p-i-n structures, multiple quantum well structures, etc. of good quality in a microcrystal level.
JP 2005-228936 A discloses a light emitting device in which a light emitting layer containing a columnar group 13 nitride semiconductor is formed between a pair of electrodes. FIG. 7 shows a schematic configuration of a light emitting device 70 using a columnar crystal (hereinafter referred to as nanocolumn) of GaN. The light emitting device 70 is formed by stacking a back electrode 72, nanocolumns 74 disposed so as to be embedded in an insulating layer 73, and a transparent electrode 75 in this order on a substrate 71. The nanocolumn 74 is GaN with a diameter of several nanometers to several tens of nanometers and is disposed in such a manner that its front end protrudes slightly from the insulating layer 73. The transparent electrode 75 and the back electrode 72 are connected to each other electrically via a power supply 76. When a voltage is applied using the power supply 76, holes are injected into the nanocolumns 74 from the transparent electrode 75 connected to the positive electrode, and electrons are injected into the nanocolumns 74 from the back electrode 72 connected to the negative electrode. When the hole and the electron injected into the nanocolumn 74 recombine in the nanocolumn 74, light of luminescence corresponding to the band gap of the p-n junction is emitted. Emitted light exits the light emitting device 70 from the transparent electrode 75 side.
JP 2005-353595 A discloses a light emitting device in which a light emitting layer containing a nanocrystal of a group 13-15 compound semiconductor, etc., a polymer hole transport layer, and an organic electron transport layer are sandwiched between a pair of electrodes. FIG. 8 shows a schematic configuration using a nanocrystal of a compound semiconductor such as GaN. The light emitting device 80 is formed by stacking an anode (transparent) 82, a polymer hole transport layer 83, a light emitting layer 84 containing a nanocrystal, an organic electron transport layer 85 and a cathode 86 in this order on the substrate 81. The light emitting layer 84 contains a compound semiconductor nanocrystal with a diameter of 3 nm to 30 nm. The anode 82 and the cathode 86 are connected electrically via a power supply 87. When a voltage is applied using the power supply 87, holes are injected into the light emitting layer 84 from the anode 82 through the polymer hole transport layer 83, and electrons are injected into the light emitting layer 84 from the cathode 86 through the organic electron transport layer 85. The hole and the electron injected into the light emitting layer 84 reach the compound semiconductor nanocrystal in the light emitting layer 84, and light of luminescence due to recombination is emitted. The emitted light exits the light emitting device 80 from the anode 82 side.
In general, holes and electrons should be injected efficiently into a particulate luminescent material such as a semiconductor particle in order to achieve efficient luminescence in LED. The conventional techniques mentioned above have the advantage of reducing the mismatch with a substrate (such mismatch is observed in the LEDs) and have the advantage of ease of enlargement, however, they have not reached a sufficient level as a display device in practical use in view of brightness and light emission efficiency.